Solvent extraction processes for the recovery of metals, and particularly uranium, from aqueous solutions, such as crude, industrial phosphoric acid solutions, produced from phosphate rock, are well known, and are taught, for example by Bailes and Long, in U.S. Pat. No. 2,859,092. Such processes comprise using various chemicals, such as alkyl phosphoric, alkyl phosphonic, or alkyl phosphitic acids, among others, to extract the uranium from the phosphoric acid.
After the uranium is recovered from the acid, the acid is returned to the acid producer for further processing, to make, for example, various types of fertilizer. However, the chemicals used in the solvent extraction process may interfere with further processing, and must be removed, increasing both the complexity and cost of the uranium recovery process.
Reverse osmosis processes are well known as means for purifying sea or waste water solutions, containing from about 100 to 5,000 parts, per million parts water, of salt or other dissolved solids, as taught by Stana, in U.S. Pat. No. 3,593,855. Reverse osmosis has also been used to remove dissolved minerals, such as iron, calcium, magnesium, manganese, and aluminum from sulfate containing, contaminated, acid mine drain waters, which have generally been pretreated by chlorination and ferrous iron oxidation, as taught by Hill et al., in U.S. Pat. No. 3,795,609.
In the Hill et al. process, a single reverse osmosis step is used. The concentrated acidic metal containing stream is then neutralized, from a pH of about 1.5 to a pH of 4.5, by addition of large quantities of lime, to form a precipitate of iron, aluminum and sulfate. This is then followed by ferrous-to-ferric oxidation, as by aeration. A final clarification step follows, to remove the concentrated sludge from the neutralized solution, which solution is recycled into the mine acid feed.
Sastri and Ashbrook, in Separation Science, 11(4), pp. 361-376, 1961, describe the use of single step reverse osmosis as the sole means to remove uranyl sulfate, UO.sub.2 SO.sub.4, from mine water feed. The metal ions separated are Ca.sup.+2, Fe.sup.+3, Al.sup.+3 and U.sup.+6, using supported, preshrunk, "tight", cellulose acetate permselective membranes, having rejection rates of between about 50% to 90% on aqueous NaCl. Such a system would have a very low purified material flux.
What is needed is a commercially feasible process, specifically adapted to concentrate the small amounts of uranium in the natural valence state, present in wet process phosphoric acid (WPA) and other type solutions. This process should not require expensive neutralization of the acid, or the addition of any chemicals that will interfere with the further processing of the crude acid solution. Ideally, and importantly, this process should also provide a phosphate solution substantially free of iron and organic impurities.